1. Field of the Invention
This invention relates to an image scanning system, and more particularly to an image scanning system in which an object image formed by an optical system is electrically scanned by the use of a photosensor array device and the image scanning output then obtained is quantized to provide quantized data on the object image.
2. Description of the Prior Art
Owing to the recent remarkable development of the semiconductor art, the so-called self-scanning type sensor array devices such as self-scanning type photo-diode arrays, CCD photosensors or CCD photo diode arrays comprising a combination of CCD and photodiodes have become commercially available at low cost and in such a trend of technological innovation, efforts have been made to utilize such type of photosensor array device as electrical image scanning means to automatically detect the distance to an object or the focus point of the optical system with respect to the object with the aid of the scanning output signal provided thereby.
For example, U.S. Pat. No. 4,004,852 proposes an electronic automatic range finding device wherein a standard field image and a reference field image containing the standard field image and being in a wider range than the standard field image are formed regarding a target object by a base line range finder type optical system and these standard and reference field images are electrically scanned by the use of a photosensor array device while, at the same time, the scanning output signals then obtained regarding the standard and reference field images are transformed into binary form (quantized) by a binary encoding circuit, whereafter the binary data are stored in shift registers so that the binary data on the standard field image stored in a shift register is compared with the binary data on the reference field image stored in a shift register, namely, the image correlation therebetween is taken, thereby detecting an image portion of the reference field image which can be regarded as being coincident with the standard field or which is most similar to the standard field and detecting the distance to the target object from the information on the alignment, namely, location, of the most similar image portion within the reference field image.
Now, where an object image is thus scanned by the use of a photosensor array device while, at the same time, the image scanning output signal then obtained is quantized to provide a quantized data on the object image and the distance to the object or the focus point of the optical system with respect to the object is to be detected on the basis of such quantized data, how to obtain quantized data exactly corresponding to the image pattern of the object is the fundamental requirement for fully ensuring the detection accuracy. That is, in such type of apparatus, no matter how high the performance of the data processing system may be, the detection accuracy thereof cannot be warranted unless the quantized image data as an input accurately represents the information on the image pattern of the object and accordingly, in such type of apparatus, the quality of the detection accuracy thereof is determined entirely by the quantization process of the image scanning output signal obtained. For example, the apparatus proposed in the aforementioned U.S. Pat. No. 4,004,852 adopts, as the method of quantization process of the image scanning output signal, a method which utilizes a saturation level of the sensor device output (this means, in case of a photosensor array device which is CCD photosensor or CCD photodiode array, an output level at which the stored charge in the photosensor portion is saturated) and a level of the sensor device output with no light impinging on the sensor device, namely, a dark level and uses 1/2 of the difference between the saturation level and the dark level as the standard level for quantizing (binary-encoding) the image scanning output signal, namely, the slice level, and wherein the level over this slice level is a logic value "1" and the level below this slice level is a logic value "0". According to this, the slice level is always fixed while, on the other hand, the level of the image scanning output is variously variable in accordance with variations in brightness of the target object and therefore, it is utterly impossible to obtain accurate quantized data on the object image and in some cases, there are only obtained data which are all "1" or all "0" and in such cases, the detection of the distance is entirely impossible.
In contrast, for example, if the peak value of the image scanning output obtained during each cycle of image scanning is detected each time and a voltage level obtained by multiplying this peak value by a certain ratio (a certain constant smaller than 1) is used as the slice level and held until the next cycle of image scanning is terminated and the image scanning output obtained during this next cycle of image scanning is quantized with the said slice level obtained during the previous cycle of image scanning being as the standard, variation in the level of the image scanning output resulting from variation in brightness of the object accompanies variation in the peak value thereof as well as variation in the slice level, thus enabling accurate quantization of the image scanning output.
On the other hand, the level of the sensor device output should desirably be always in a predetermined level range to facilitate the subsequent processing and therefore, for variation in brightness of the object, it is the practice to suitably adjust the integration time of the image signal of the sensor device (in case of a CCD photosensor or a CCD photodiode array, the charge storage time in the photosensor portion), namely, to shorten this integration time when the object is bright and extend the integration time when the object is dark, thereby adjusting the level of the obtained sensor device output so that it is always within a predetermined level range irrespective of the variation in brightness of the object. The most rational and simplest method as the method of adjusting the integration time is to detect the level of the sensor device output and vary the timing for reading out the sensor device output on the basis of the result of the detection, but here the following inconvenience will occur when the integration time of the sensor device is suitably adjusted on the basis of the information on the brightness of the object while, on the other hand, as the method of quantizing the image scanning output, a slice level is determined on the basis of the peak value of the image scanning output and the quantization of the image scanning output obtained during the next cycle of image scanning is effected with the said slice level as the standard.
That is, for example, assuming that the integration time of the sensor device has been changed, the scanning output whose level has been properly adjusted so as to be within said predetermined level range by the change in the integration time is obtained in at least the next cycle, or in some cases, the next cycle (namely, the second cycle as counted from the point of time whereat the integration time was changed) of image scanning while, on the other hand, the slice level for the scanning output obtained with the integration time changed is determined on the basis of the peak value of the scanning output obtained during the previous cycle of image scanning. In other words, the scanning output before the integration time is changed, and accordingly, the scanning output before the level thereof is properly adjusted so as to be within said predetermined level range and therefore, in this case, the good scanning output obtained with the integration time changed is quantized by the slice level set on the basis of the peak value of the improper scanning output before the integration time is changed and thus, it is utterly impossible to obtain accurate quantized image data and after all, the image scanning of this cycle, namely, the cycle immediately succeeding to the change in the integration time becomes quite useless and this also leads to much loss of time. Also, this becomes a great problem where the change in the integration time is effected frequently. In addition, in the above-described method of quantization process, the aforementioned peak value is not yet obtained during the first cycle of image scanning and accordingly, the foregoing inconvenience also takes place with regard to the first cycle of image scanning.